The invention relates to a circuit for controlling a cathode ray tube (CRT) and a television receiver equipped with such a circuit. A further aspect of the invention is related to control the driving signals for a CRT.
For a television receiver the maximum brightness, which can be represented on the viewing screen of the CRT or picture tube, the so-called light at peak white is an important quality characteristic of the image reproduction. In practice, the light at peak white is essentially limited firstly, by the control range or the saturation limit of the RGB output stages controlling the picture tube and, secondly, by the maximum permissible drive of the picture tube.
In order to achieve the highest possible brightness, it is therefore necessary to set the drive of the picture tube, that is to say the amplification of the RGB signals controlling the picture tube, as high as possible in the direction of white.
However, when the video output stages are driven in this way they may occasionally reach the saturation limit of the amplifiers in the output stages. Substantial tolerances such as, for example, the cut-off calibration, the transparency of the picture tube glass and fluctuations in the input signal, the dark current regulation compensating the ageing of the picture tube as well as customer settings such as brightness, contrast and colour saturation are responsible for exceeding the saturation limit.
If an amplifier of a video output stage reaches the saturation limit, so-called smears appear on the viewing screen in the corresponding colour in the form of an extension of a pixel or of an image area in the line direction. These smears occur primarily for the colour red, because the electron gun for the colour red of the picture tube requires the highest drive because the red phosphor is least sensitive.
In order to avoid such saturation phenomena in the case mentioned where tolerances sum up in an unfavourable way, it would therefore be necessary to observe a certain safety distance towards full output of the output amplifiers. However, this means in turn that the theoretically maximum possible drive to full output is generally not reached. Consequently, it is not possible to reach the maximum brightness of the picture tube.
U.S. Pat. No. 6,057,887 suggests a circuit to alleviate the problems described above. In the known circuit the output colour signals are limited by Zener diodes to a threshold value that avoids the occurrence of smears on the screen. However, in practice it has been noted that the use of the known circuit in connection with the video amplifier IC TEA 5101 manufactured by ST has led to enhanced smearing effects due to xe2x80x9clatchingxe2x80x9d effects inside the video amplifier IC.
Based on these observations it is desirable to provide a circuit, which reliably avoids all the described smears.
The invention provides a circuit for controlling a cathode ray tube comprising a source of input colour signals (Uin(R), Uin(G), Uin(B)) for the primary colours of the cathode ray tube. Output amplifiers are coupled to the source of colour input signals and to electron guns of the cathode ray tube. Finally, limiting means are provided to limit the colour input signals to a predetermined threshold value.
In an advantageous embodiment of the inventive circuit the limiting means comprise a diode biased in reverse direction. Preferably a voltage source provides the bias voltage of the diode. The voltage source may be realized by a voltage divider stabilized with a Zener diode.
In a preferred embodiment of the invention the same voltage source utilized to provide a reference voltage (Uref) for the output amplifier is also utilized to provide the biasing voltage for the diode.
It is advantageous to connect the limiting means to a transistor providing a low impedance voltage source in case the input signal exceeds the threshold voltage.
Further advantages of the invention become apparent when reading the detailed description of the embodiments of the invention.